A highly concentrated chemical or cleaning agent is used for washing and surface finishing of electronic components, and a large amount of pure water is used for rinsing of them. Then, development of a high-level water treatment technique is desired so as to improve water quality of pure water or recycling efficiency of drain water.
In a step of producing ultra-pure water, a biological treatment or physicochemical treatment is widely conducted as a method for removing a TOC (total organic carbon) component from water to be treated.
For example, there is a method in which a TOC component is removed by biologically treating organic-matter-containing drain water and then the biologically treated water is purified through a reverse osmosis membrane (RO membrane) (e.g., Patent Document 1).
In addition, for the physicochemical treatment, there are methods such as the following (1) to (3).
(1) A method for removing organic matter by passing organic-matter-containing drain water directly through an RO membrane separation device.
(2) A method for thermolysis of organic matter by adding an oxidizing agent to organic-matter-containing drain water.
(3) A method for degrading organic matter by irradiating organic-matter-containing drain water with ultraviolet (UV) light (e.g., Patent Document 2).
As Patent Document 1 indicates, for a method for passing water through an RO membrane separation device following biological treatment of organic-matter-containing drain water, there is a problem that the membrane flux decreases because biological metabolites generated in the removal of organic matter by microorganisms clog the membrane surface of a RO membrane. Additionally, the power consumption of a water-feeding pump of an RO membrane separation device is large.
There is a problem with the above methods (1) to (3) in that each of them has large energy consumption.
In method (1), the power for driving a pressure pump for supplying water to an RO membrane separation device is large. When organic-matter-containing drain water passes directly through the RO membrane separation device, this organic-matter-containing drain water having a high TOC concentration readily results in biofouling in the RO membrane separation device. In addition, when the organic matter in the organic-matter-containing drain water is persistent organic matter, an example of which being a low-molecular-weight nitrogen compound (e.g., urea), the removal efficiency is extremely low.
Method (2) requires a large amount of heat energy in a thermolysis device at the time of heating, by using steam, etc., the organic-matter-containing drain water to which an oxidizing agent has been added.
Method (3) requires a large amount of power for UV irradiation. Besides, even for this UV irradiation, the degradation efficiency is extremely low when the organic matter in the organic-matter-containing drain water is persistent organic matter.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-336886
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-185587